Recoverable articles have found wide use for environmental sealing because of the ease with which they can be installed, the strength of seal that can be obtained, and the wide range of size of substrates with which any one design can be used. Recovery can be initiated by various means, although heat-recovery is at present preferred since heat-recoverable products are simple to produce and do not require specialized equipment to install. The article is simply placed over the substrate to be protected and heated which causes it to recover, generally by shrinking, into close conformity with the substrate.
A heat-recoverable article (an article the dimensional configuration of which may be made substantially to change when subjected to heat treatment) usually will recover towards an original shape from which it has previously been deformed but the term "heat-recoverable", as used herein, also includes an article which, on heating, adopts a new configuration, even if it has not been previously deformed.
In their most common form, such articles comprise a heat-shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Pat. Nos. 2,027,962; 3,086,242 and 3,597,372. As is made clear in, for example, U.S. Pat. No. 2,027,962 the original dimensionally heat-stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded, whilst hot, to a dimensionally heat-unstable form but, in other applications, a preformed dimensionally heat stable article is deformed to a dimensionally heat unstable form in a separate stage.
In the production of heat recoverable articles, the polymeric material may be cross-linked at any stage in the production of the article that will enhance the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises shaping the polymeric material into the desired heat-stable form, subsequently cross-linking the polymeric material, heating the article to a temperature above the crystalline melting point or, for amorphous materials the softening point, as the case may be, of the polymer, deforming the article and cooling the article whilst in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape.
In other articles, as described, for example, in British Pat. No. 1,440,524, an elastomeric member such as an outer tubular member is held in a stretched state by a second member, such as an inner tubular member, which, upon heating weakens and thus allows the elastomeric member to recovery.
There are instances where the substrate to be environmentally sealed has no accessable end (for example a cable or pipe which cannot conveniently be severed) or where the substrate is situated in a confined space (such as a man-hole). In such cases a tubular article cannot be slid over an end of the substrate, and to overcome the problem wrap-around sleeves have been developed. A wrap-around sleeve is simply a sleeve which can be installed around a substrate with no free end and then secured in the wrapped-around configuration. The sleeve may be provided with closure members adjacent apposing edges, which interlock or which are held together by a further member.
The commonest method of heating such sleeves to cause recovery is by means of a propane torch or a hot-air gun. A propane torch has the advantage of being portable and of providing the correct temperature for the larger sleeves most commonly used, although uniform heat application in confined spaces is difficult, and its use is prohibited where flammable materials are present. Hot-air guns are rarely a good substitute since they are unable to deliver sufficent heat at the right temperature.
In order to avoid the disadvantages of the use of a flame, electrical heating systems have been developed. As well as the basic advantage of no flame, an electrical system can be compact, if operated at low voltages can totally avoid any danger of sparking, and can incorporate self-regulation making installation very simple.
Systems for electrical heating of recoverable sleeves can be classified as two general types: firstly, an electrical heater can be secured in thermal contact with a recoverable sleeve, and secondly the material of the sleeve may itself constitute part of the electrical circuit. In the first of these types electrical heating wires or sheet may be bonded to or embedded in the sleeve so that the sleeve becomes hot simply through conduction.
This idea is disclosed in published European Patent Application No. 38659 to UBE Industries Ltd. A heat-shrinkable cover sheet has branched tongue-like portions at its end, which can slot and bond together. The ends of the sheet and the central part are separately provided with electrical heating wires arranged in a zig-zag pattern. The wires simply terminate at the edge of the sheet for connection to a power source.
The second type mentioned above employs a special recoverable sleeve which itself has the ability to become hot when subjected to electrical power. The requirement here is for a material which is electrically resistive, is capable of being made heat-recoverable, and is flexible. Conductive polymers, such as those made by loading polyolefins with carbon black, have these characteristics and can be made self-regulating by proper choice of the carbon content. Self-regulation results from the material having a positive temperature coeficient of resistance (PTC) so that when the material reaches a certain temperature its electrical resistance rises, thus sharply cutting off power input and preventing further heating. This feature is of great benefit since it allows simple power supplies without thermostats or other controls to be used and it makes installation of the sleeve less craft-sensitive. The heating characteristics may be improved by combining a layer of PTC material with a layer of constant wattage (CW) material in such a way that current flows through both: this can lead to reduced current in-rush on initial connection of the power, and to a greater heating capacity. For a fuller discussion on electrically heatable polymers the reader is directed to U.S. Pat. No. 4,177,376, which although concerned mainly with heaters describes the electrical properties of the class of polymers commonly used to make recoverable sleeves.
A commercially available article embodying heat-recovery and conductive polymer self-heating is a tape marketed under the Raychem trade mark Autowrap. This tape is about 10 cm wide and has an electrical conductor running adjacent each longitudinal edge. It is wrapped spirally around a substrate to be covered, and when powered through the two conductors it shrinks longitudinally thereby tightening onto the substrate. A certain degree of care is required when wrapping this tape around the substrate since sufficient overlap between adjacent turns must be provided to prevent leak paths remaining. Careful wrapping can be difficult in some circumstances when the substrate has a significant variation in cross-sectional size along its length.